Final answer:
The class within Phylum Mollusca referred to as "Monoplacophora" includes marine mollusks with a single cap-like shell and a simple body plan. They were once thought extinct until the discovery of Neopilina galatheae in 1952. This class is crucial for understanding molluscan evolutionary relationships.
Step-by-step explanation:
The class within Phylum Mollusca that is referred to as "Monoplacophora" is a group of marine mollusks that were once thought to be extinct. This unique class is known for possessing a single, cap-like shell that covers their dorsal side. Monoplacophorans have a simple anatomy with notable features such as a looped digestive system, multiple pairs of excretory organs, and multiple gills which distinguishes them from other mollusks.
Monoplacophora was rediscovered in 1952 with the finding of Neopilina galatheae, which expanded the known diversity of mollusks. These organisms have a basic body plan indicative of early molluscan evolution, making them a significant subject for studying the evolutionary relationships within Mollusca.
Each Monoplacophoran is equipped with a unique radula, paired gonads, and multiple gills situated between the foot and the mantle's edge. Its shell morphology can range from circular to ovate. The rediscovery of Monoplacophora provides an invaluable peek into the ancient molluscan lineage and underline the diversity and adaptability within the phylum.